Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $35.43   12.585%   $39.8889   $39.89   $39.90   $39.90  
  Purchase #2 $102.48   4.699%   $107.2955   $107.30   $107.30   $107.30  
  Purchase #3 $129.51   10.652%   $143.3054   $143.31   $143.30   $143.30  
  Purchase #4 $110.64   3.158%   $114.134   $114.13   $114.15   $114.10  
  Purchase #5 $169.72   13.282%   $192.2622   $192.26   $192.25   $192.30  
  Purchase #6 $199.45   13.494%   $226.3638   $226.36   $226.35   $226.40  
  Purchase #7 $152.41   5.736%   $161.1522   $161.15   $161.15   $161.20  
  7 purchase totals = $899.64       $984.4020   $984.40   $984.40   $984.50